1. Field of the Invention
The invention relates to selection of material for a fuse/safety switch in a halogen incandescent lamp. The fuse includes a metallic conductor that operates with no degradation during normal operation of the lamp and will burn out, oxidize or fail to conduct during gas changes when the outer bulb is broken. The halogen incandescent lamp includes a burner having a filament with one or both ends that are extended outside the burner through its press seals. More particularly, the extended filament ends outside the burner act as fuses or safety switches when the burner is mounted in a sealed outer envelope. The fuse/safety switch construction renders the lamp inoperable upon breach of the outer envelope and eliminates the fire hazard associated with the operation of the exposed halogen burner.
2. Discussion of the Prior Art
Safety of lamps is paramount in lamp design to protect the user from lethal and potentially destructive sources that may be present when a lamp is partially fractured. In lamps consisting of a light emitting and self contained sealed inner tube, such as a halogen incandescent burner, where electrical conduction is through an outer bulb plus base assembly, the lamp may be partially broken and still operational.
Halogen incandescent lamps consisting of an inner xe2x80x9chalogen burnerxe2x80x9d within a sealed outer have been introduced and are more efficient than conventional incandescent lamps.
FIG. 1 shows a known halogen incandescent burner or capsule having a tungsten filament 10 with a barrel 16 between a pair of tungsten legs 18, and a xe2x80x9cdouble endedxe2x80x9d quartz envelope 20, with a pinch seal or press 22 at each end. The barrel 16 is located in a central or burner cavity 24, and the coil ends or legs 18 extend into the presses 22 and are each welded to one end of a molybdenum foil 26. Molybdenum current supply leads 28 are welded to the other end of the respective molybdenum foils 26 and extend out of the presses 22.
To facilitate welding of the tungsten coiled legs 18 to the molybdenum foils 26, a small metal foil (platinum) 26A may be placed between the tungsten coil legs 18 and the molybdenum foil 26. The press 22 contains the molybdenum foil 26, the platinum foil 26A, as well as the ends of the respective tungsten coil legs 18 and the molybdenum current supply leads 28. The molybdenum foils 26 are required in the quartz envelope 20 to create a gas-tight seal in the presses 22 over the operating temperatures of the burner 5. Other burners use mechanical clamps instead of molybdenum foils to electrically attach the tungsten coil legs to the molybdenum current supply leads, where the clamps are located in the press seals and/or the central/burner cavity as described in U.S. patent application Ser. No. 09/606,396, which is incorporated herein by reference.
FIG. 2 shows the filament 10 used in the halogen burner shown in FIG. 1. The coiled-coil filament 10 has a primary coil 14 and a secondary coil 16. The filament 10 is formed with a tungsten wire 12 wound on a primary mandrel having a diameter on the order of 80-150 xcexcm to form the primary coil 14 having an external diameter on the order of 100-300 xcexcm. The primary coil 14 is wound on a secondary mandrel having a diameter on the order of 300-800 xcexcm to form the secondary coil 16 which forms the barrel 16. The secondary mandrel is retracted or dissolved, and the primary mandrel is then removed in whole or in part by dissolving. U.S. Pat. No. 4,132,922 discloses a double-ended burner having a U-shaped fuse and a so-called retained mandrel coil. Portion of the U-shaped fuse is embedded in the press seals while another portion is in the burner.
Double-ended quartz burners are marketed in thin-glass outers, such as blown glass reflectors, decorative outers and the like for general lighting applications. Double-ended quartz burners or capsules 5 with coil legs 18 extending into the presses 22, as shown in FIG. 1, have passive extinction of electric arc at end-of-life.
There is a need to eliminate non-passive failures in hard-glass halogen burners to enable marketing of thin-glass outer lamps containing the hard-glass burner. Further, there is a need to reduce the cost of lamps and to increase efficiency of manufacture thereof. There is also a need to eliminate molybdenum foils or clamps in the burner as well as eliminating the need for additional elements to form a safety switch or fuse in order to reduce lamp cost, simplify manufacture thereof, and provide a more robust lamp, while preventing lamp operation when the outer lamp envelope is breached.
Further, in a lamp having a self-contained, light emission source mounted in and electrically connected through a sealed outer, there is a need for a safety switch or fuse that will disable operation of the inner arc tube when the hermetically sealed outer is broken. There is also a need to disable power to inner arc tube at the time the hermetic seal is fractured and power applied to the lamp. A further need is to disable electrical conduction of high-voltage differences within the inner volume conductive surfaces prior to application of high current. There is also a need to achieve these safety improvements in lamps with vacuum or sub-atmospheric pressure, such as anaerobic gas for example between the inner and the outer. There is a further need to reduce the cost of double ended burners, such as made from quartz. There is also a need for reduction of stress within the press seal and metal conductor interface.
There is a further need for improvement of manufacturing efficiency, and dealing better with exhaust tubes, such as selling the exhaust tube with the lamp. There is a need for a protective layer over the fusing material to prevent ignition of nearby material in a xe2x80x9ccheese clothxe2x80x9d test. There is also a need for an arc tube design with a dominant passive failure mechanism at the end-of-life.
According to the invention, a halogen incandescent capsule or burner, which may be double-ended or single-ended, has an envelope, such as a hard-glass or quartz envelope, forming an inner/burner cavity. Illustratively, the burner is surrounded by an external envelope attached to a base to form a lamp. At least one end of the burner has a press. A filament, e.g. a tungsten filament, is included in the inner cavity. At least one leg of the filament is modified, e.g., stretched, extended, uncoiled for a coiled filament or never wound, to extend out of the press for electrical attachment, by welding for example, to lamp current supply leads in an outer/lamp cavity between the burner and the external envelope of the lamp. For a double-ended burner, one or both legs of the filament may be modified to extend out of one or both of the presses, while for a single-ended burner, one of the legs of the filament is modified to extend out of the press. The lamp cavity is evacuated and filled with a non-oxidizing gas at less than one atmosphere of pressure.
The tungsten wires that extend out of the burner and are connected to the current supply leads in the outer cavity act as fuses or safety switches, thus eliminating the need for additional elements to form a fuse or to form lead wires and foils in the presses.
When the filament fails at end-of-life, the arc is extinguished passively with disintegration of the filament leg in the inner cavity and/or near the inside surface of the presses.
According to a further aspect of the invention, the filament has a primary coil, where the primary coil of the filament leg is modified so that the leg portion in the press is straight or has an increased pitch. The extended coil leg reduces the extinction time and electric arc energy at the end-of-life due to reduced linear wire density near the pinch. Further, the portion of the stretched-out tungsten leg outside the burner, i.e., in the outer/lamp cavity acts as a fuse or safety switch. In particular, upon breach of the outer or external lamp envelope and with power applied to the lamp, this extended leg portion disintegrates thus acting as an open fuse or switch and preventing continued operation of the burner upon breach of the outer. This eliminates an otherwise fire hazard due to the high temperature of the operating burner.
Modifying, e.g., stretching out, the coil leg enables a robust to attachment of the tungsten wire, e.g., by welding, to the lead wire of the lamp. This provides an economical manufacture of the lamp having a fuse or safety switch, by simple manufacture of the burner having extended tungsten leads for attachment to the lamp current supply lead wires. Thus, clamping the lamp lead wires on the primary winding is eliminated which requires a tight tolerance gap within the molybdenum clamp, which in turn, eliminates strain in the clamped leg and fractures of the coil leg. Attachment of the modified coil leg to the lamp lead negates the requirement of changeover time between wattages at the mount machine.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.